I have been banging my head for days and think I have finally narrowed things down. Following the HowTo listed in the title everything appeared to be going fine until I tried to login to Horde. I am down to this:
if I login to horde with user test_user@domain.net and watch the logs I see it is not passing the domain.net through to authenticate. I then went into MySQL and removed domain.net from that user. It now authenticates fine, however when trying to send an email I get the following error: "There was an error sending your message: Validation failed for: test_user@.SYNTAX-ERROR"
Honestly there is only one domain being used and always will be that way I would just assume users not have to put in the domain.net. Is there a way to knock out both problems at once?

If I remove the domain from the users name in the horde_users SQL table it works. Then of course horde will not send mail because it doesn't have the domain set, so when I compose email it comes up but when you click send it fails. Watching the logs I can't get saslauthd to pass the realm.